This invention relates to an antitheft device which is operable to photoelectrically detect cutting of a locking cable (lock wire) including an optical fiber and electric conductors, for giving an alarm, and more particularly to improvements in the locking cable.
An antitheft device for vehicles such as motorcycles is known from Japanese Provisional Patent Publication (Kokai) No. 58-62791 by the assignee of the present application, which uses a locking cable, through which an optical fiber and electric conductors extend. This antitheft device is used in such a manner that a vehicle, which is to be protected from theft, is tied to a solid structure by means of the locking cable, while a photo signal is transmitted from a light emission element to a light receiving element through the optical fiber. When the locking cable is cut in two to cause interruption of the transmission of the photo signal from the light emission element to the light receiving element, the cutting of the locking cable is detected to actuate a warning device to give warning.
Such locking cable used in the conventional antitheft device comprises a core, an inner layer, and an outer layer, for instance. The core is formed of an optical fiber extending longitudinally along the axis and covered with a protective member made of a synthetic resin. The inner layer is formed of two electric conductors wound around the outer peripheral surface of the core, together with a plurality of auxiliary cords having almost the same diameter as the electric conductors, and the electric conductors and auxiliary cords are covered with a protective member made of a synthetic resin. The outer layer is formed of a plurality of strands each formed of twisted steel wires and wound around the outer peripheral surface of the inner layer, and covered with a protective member made of a synthetic resin. The inner layer including the electric conductors is firmly held between the core and the outer layer.
With the above arrangement, the electric conductors in the inner layer have very small freedom to move in the twisting direction or in the opposite direction. When a bending force is given to the locking cable with such a magnitude and through such an angle as to exceed the freedom, the electric conductors will be cut. Therefore, the flexibility of the locking cable is set such that the locking cable can be bent only by a force and through an angle not exceeding the freedom, which imposes a limitation upon the minimum winding diameter of the locking cable when it is accommodated in the form of a coil within an accommodating space of the vehicle.
The locking cable constructed as above is connected to the antitheft device with a plug or plugs provided at an end or ends of the locking cable to be locked and unlocked with respect to the antitheft device by a key. The light emission element and the optical fiber have their ends disposed in face-to-face relation with a predetermined space therebetween within the plug and held in place by a holder provided in the plug. The electric conductors are held between the outer peripheral surface of the holder and the inner peripheral surface of the plug. Therefore, neither of the optical fiber and the electric conductors can be moved axially in the plug. This also imposes a limitation upon the minimum winding diameter of the locking cable when it is wounded into a coil.